Multiple cell battery charger configured with a parallel topology

ABSTRACT

A multiple cell battery charger with a parallel topography is disclosed which requires fewer active components than known battery chargers and protects the battery cells from overcharge and discharge. The charger that includes a regulator for providing a regulated source of direct current (DC) voltage to the battery cells to be charged. Each battery cell is connected in series with a switching device, such as a field effect transistor (FET) and optionally a current sensing device. In a charging mode, the serially connected FET conducts, thus enabling the battery cell to be charged. Battery voltage is sensed by a microprocessor. When the microprocessor senses that the battery cell is fully charged, the FET is turned off, thus disconnecting the battery cell from the circuit. Since the battery cell is disconnected from the circuit, no additional active devices are required to protect the battery cell from discharge.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a battery charger and moreparticularly, to a battery charger for charging two or more rechargeablebattery cells using a parallel battery charger topology, which, as aresult uses a reduced number of active components resulting in arelatively less expensive battery charger and at the same time providesthe ability to independently control the charging of each of the batterycells.

2. Description of the Prior Art

Various portable devices and appliances are known to use multiplerechargeable battery cells, such as AA and AAA battery cells. In orderto facilitate charging of the battery cells for such multiple cellappliances, multiple cell battery chargers have been developed. Bothparallel and series topologies are known for such multiple cell batterychargers. For example, U.S. Pat. Nos. 5,821,733 and 6,580,249, as wellas published U.S. Patent Application U.S. 2003/0160593, disclosemultiple cell battery chargers configured in a series topology. U.S.Pat. Nos. 6,034,506 and 6,586,909 as well as published U.S. PatentApplication U.S. 2003/0117109 A1 disclose battery chargers configured ina parallel topology.

In such multiple cell battery chargers configured in a series topology,a series charging current is applied to a plurality of serially coupledbattery cells. Because the internal resistance and charge on theindividual cells may vary during charging, it is necessary with suchbattery chargers to monitor the voltage across and/or temperature ofeach cell in order to avoid overcharging any of the serially connectedcells. In the event that an over-voltage condition is sensed, it isnecessary to shunt charging current around the cell to preventovercharging of any of the individual serially connected cells. Thus,such multiple cell battery chargers normally include a parallel shuntaround each of the serially connected cells. As such, when a batterycell becomes fully charged, additional charging current is thus shuntedaround the cell to prevent overcharging and possible damage to the cell.In addition, it is necessary to prevent discharge of such seriallyconnected battery cells when such cells are not being charged.

Various embodiments of a multiple cell battery charger configured with aserial charging topography are disclosed in the '733 patent. In oneembodiment, a Zener diode is connected in parallel across each of theserially connected battery cells. The Zener diode is selected so thatits breakdown voltage is essentially equivalent to the fully-chargedvoltage of the battery cell. Thus, when any of the cells become fullycharged, the Zener diode conducts and shunts current around that cell toprevent further charging of the battery cell. Unfortunately, the Zenerdiode does not provide relatively accurate control of the switchingvoltage.

In an alternate embodiment of the battery charger disclosed in the '733patent, a multiple cell battery charger with a series topology isdisclosed in which a field effect transistors (FET) are used in place ofthe Zener diodes to shunt current around the battery cells. In thatembodiment, the voltage across each of the serially connected cells ismonitored. When the voltage measurements indicate that the cell is fullycharged, the FET is turned on to shunt additional charging currentaround the fully charged cell. In order to prevent discharge of batterycells, isolation switches, formed from additional FETs, are used. Theseisolation switches simply disconnect the charging circuit from theindividual battery cells during a condition when the cells are not beingcharged.

U.S. Pat. No. 6,580,249 and published U.S. Patent Application No. U.S.2003/01605393 A1 also disclosed multiple cell battery chargersconfigured in a serial topology. The multiple cell battery chargersdisclosed in these publications also include a shunt device, connectedin parallel around each of the serially coupled battery cells. In theseembodiments, FETs are used for the shunts. The FETs are under thecontrol of a microprocessor. Essentially, the microprocessor monitorsthe voltage and temperature of each of the serially connected cells.When the microprocessor senses that the cell voltage or temperature ofany cell is above a predetermined threshold indicative that the cell isfully charged, the microprocessor turns on the FET, thus shuntingcharging current around that particular battery cell. In order toprevent discharge of the serially connected cells when no power isapplied to the battery charger, blocking devices, such as diodes, areused.

Although such multiple cell battery chargers configured in a seriestopology are able to simultaneously charge multiple battery cellswithout damage, such battery chargers am as discussed above, not withoutproblems. For example, such multiple cell battery chargers require atleast two active components, namely, either a Zener diode or a FET as ashunt and either a FET or diode for isolation to prevent discharge. Theneed for at least two active devices drives up the cost of such multiplebattery cell chargers.

As mentioned above, U.S. Pat. Nos. 6,034,506 and 6,586,909, as well asU.S. Published Patent Application No. U.S. 2003/0117109, disclosemultiple cell battery chargers configured in a parallel topology. U.S.Pat. No. 6,586,909 and published U.S. Application No. U.S. 2003/0117109disclose a multiple cell battery charger for use in charging industrialhigh capacity electrochemical batteries. These publications disclose theuse of a transformer having a single primary and multiple balancedsecondary windings that are magnetically coupled together by way of aninduction core. Each battery cell is charged by way of a regulator,coupled to one of the multiple secondary windings. While such aconfiguration may be suitable for large industrial applications, it ispractically not suitable for use in charging appliance size batteries,such as, AA and AAA batteries.

Finally, U.S. Pat. No. 6,034,506 discloses a multiple cell batterycharger for charging multiple lithium ion cells in parallel. Inparticular, as shown best in FIG. 3 of the '506 patent, a plurality ofserially connected lithium ion battery cells are connected togetherforming a module. Multiple modules are connected in series and inparallel as shown in FIG. 2 of the '506 patent. Three isolation devicesare required for each cell making the topology disclosed in the '506patent even more expensive to manufacture than the series batterychargers discussed above. Thus, there is a need for a battery chargerwhich requires fewer active components than known battery chargers andis thus less expensive to manufacture.

SUMMARY OF THE INVENTION

Briefly, the present invention relates to a multiple cell batterycharger configured in a parallel topology. In accordance with animportant aspect of the invention, the multiple cell battery chargerrequires fewer active components than known battery chargers, while atthe same time preventing overcharge and discharge of the battery cells.The multiple cell battery charger in accordance with the presentinvention is a constant voltage battery charger that includes aregulator for providing a regulated source of direct current (DC)voltage to the battery cells to be charged. In accordance with thepresent invention, the battery charger includes a pair of batteryterminals that are coupled in series with a switching device, such as afield effect transistor (FET} and optionally a battery cell chargingcurrent sensing element, forming a charging circuit. In a charging mode,the serially connected FET conducts, thus enabling the battery cell tobe charged. The FETs are controlled by a microprocessor that alsomonitors the battery cell voltage and optionally the cell temperature.When the microprocessor senses a voltage or temperature indicative thatthe battery cell is fully charged, the FET is turned off, thusdisconnecting the battery cell from the circuit. Once the battery cellis disconnected from the charger by the FET, additional active devicesare not required to isolate the battery cell to prevent the batterycharger circuit from discharging the battery cell. As such, a singleactive device such as the FET, provides multiple functions withoutrequiring additional active devices. Accordingly, the battery charger inaccordance with the present invention utilizes fewer active componentsand is thus less expensive to manufacture than known battery chargersconfigured with a serial topography.

DESCRIPTION OF THE DRAWING

These and other advantages of the present invention will be readilyunderstood with reference to the following specification and attacheddrawing wherein:

FIG. 1 is a schematic diagram of the battery charger in accordance withthe present invention.

FIG. 2 is a graphical illustration of the voltage, pressure, and/ortemperature charging characteristics as a function of time as anexemplary NiMH battery.

FIGS. 3A-3E illustrate exemplary flow-charts for the battery charger forthe present invention.

DETAILED DESCRIPTION

The present invention relates to a constant voltage multiple cellbattery charger configured to charge multiple battery cells connected inparallel defining a parallel topology. The battery charger, generallyidentified with the reference 20, includes a power supply 22 and aregulator 24. In an AC application, the power supply 22 is configured toreceive a source of AC power, such as 120 volts AC, and convert it to anon-regulated source of DC power by way of a bridge rectifier (notshown), for example. or other device, such as a switched mode powersupply. In DC applications, the power supply 22 may simply be aunregulated source of DC, for example in the range of 10 to 16 volts DC,such as a vehicular power adapter from an automobile. The unregulatedsource of DC power from the power supply 22 may be applies to, forexample to a regulator, such as, a DC buck regulator 24, which generatesa regulated source of DC power, which, in turn, is applied to thebattery cells to be charged.

The regulator 24 may be an integrated circuit (IC) or formed fromdiscrete components. The regulator 24 may be, for example, a switchingtype regulator which generates a pulse width modulated (PWM) signal atits output. The regulator 24 may be a synchronous buck regulator 24, forexample, a Linear Technology Model No. LTC 1736, a FairchildSemiconductor Model No. RC5057; a Fairchild Semiconductor Model No.FAN5234; or a Linear Technology Model No. LTC1709-85 or others.

The output of the regulator 24 may optionally be controlled by way of afeedback loop. In particular, a total charging current sensing device,such as a sensing resistor R11, may be serially coupled to the output ofthe regulator 24. The sensing resistor R11 may be used to measure thetotal charging current supplied by the regulator 24. The value of thetotal charging current may be dropped across the sensing resistor R11and sensed by a microprocessor 26. The microprocessor 26 may beprogrammed to control the regulator 24, as will be discussed in moredetail below, to control the regulator 24 based on the state of chargeof the battery cells being charged.

As shown in FIG. 1, the battery charger 20 may optionally be configuredto charge four battery cells 28, 30, 32, and 34. As shown, these batterycells 28, 30, 32 and 34 are electrically coupled to corresponding pairsof battery terminals: T₁ and T₂; T₃ and T₄; T₅ and T₆; and T₇ and T₈,respectively. However, the principles of the present invention areapplicable to two or more battery cells.

Each battery cell 28, 30, 32 and 34 is serially connected to a switchingdevice, such as a field effect transistor (FET) Q12, Q13, Q14 and Q15.More particularly, the source and drain terminals of each of the FETsQ12, Q13, Q14 and Q15 are serially connected to the battery cells 28,30, 32 and 34. In order to sense the charging current supplied to eachof the battery cells 28, 30, 32 and 34, a current sensing devices, suchas the sensing resistors R37, R45, R53, R60, may be serially coupled tothe serial combination of the FETs Q12, Q13, Q14 and Q15; and the pairsof battery terminals, T₁ and T₂; T₃ and T₄; T₅ and T₆; and T₇ and T₈.The serial combination of the battery terminals T₁ and T₂; T₃ and T₄; T₅and T₆; and T₇ and T₈; FETs Q12, Q12, Q14 and Q15; and the optionalcharging current sensing devices R37, R45, R53 and R60, respectively,form a charging circuit for each battery cell 28, 30, 32 and 34. Thesecharging circuits, in turn, are connected together in parallel.

The charging current supplied to each of the battery cells 28, 30, 32and 34 can vary due to the differences in charge, as well as theinternal resistance of the circuit and the various battery cells 28, 30,32 and 34. This charging current as well as the cell voltage andoptionally the cell temperature may be sensed by the microprocessor 26.In accordance with an important aspect of the present invention, themultiple cell battery charger 20 may be configured to optionally sensethe charging current and cell voltage of each of the battery cells 28,30, 32 and 34, separately. This may be done by control of the seriallyconnected FETS Q12, Q13, Q14 and Q15. For example, in order to measurethe cell voltage of an individual cell, such as the cell 28, the FET Q12is turned on while the FETs Q13, Q14 and Q15 are turned off. When theFET 12 is turned on, the anode of the cell 28 is connected to systemground. The cathode of the cell is connected to the Vsen terminal of themicroprocessor 26. The cell voltage is thus sensed at the terminal Vsen.

As discussed above, the regulator 24 may be controlled by themicroprocessor 26. In particular, the magnitude of the total chargingcurrent supplied to the battery cells 28, 30, 32 and 34 may be used todetermine the pulse width of the switched regulator circuit 24. Moreparticularly, as mentioned above, the sensing resistor R11 may be usedto sense the total charging current from the regulator 24. Inparticular, the charging current is dropped across the sensing resistorR11 to generate a voltage that is read by the microprocessor 26. Thischarging current may be used to control the regulator 24 andspecifically the pulse width of the output pulse of the pulse widthmodulated signal forming a closed feedback loop. In another embodimentof the invention, the amount of charging current applied to theindividual cells Q12, Q13, Q14 and Q15 may be sensed by way of therespective sensing resistors R37, R45, R53 and R60 and used for controlof the regulator 24 either by itself or in combination with the totaloutput current from the regulator 24. In other embodiments of theinvention, the charging current to one or more of the battery cells 28,30, 32 and 34 may be used for control.

In operation, during a charging mode, the pulse width of the regulator24 is set to an initial value. Due to the differences in internalresistance and state of charge of each of the battery cells 28, 30, 32and 34 at any given time, any individual cells which reach their fullycharged state, as indicated by its respective cell voltage, as measuredby the microprocessor 26. More particularly, when the microprocessor 26senses that any of the battery cells 28, 30, 32 or 34 are fully charged,the microprocessor 26 drives the respective FETs Q12, Q13, Q14, or Q15open in order to disconnect the respective battery cell 28, 30, 32 and34 from the circuit. Since the battery cells are actually disconnectedfrom the circuit no additional active devices are required to protectthe cells 28, 30, 32 and 34 from discharge. Thus, a single active deviceper cell (i.e., FETs Q12, Q13, Q14 and Q15) are used in place of twoactive devices normally used in multiple cell battery chargersconfigured with a serial topology to provide the dual function ofpreventing overcharge to individual cells and at the same timeprotecting those cells from discharge.

As mentioned above, the charging current of each of the battery cells28, 30,32 and 34 is dropped across a sensing resistor R37, R45, R53 andR60. This voltage may be scaled by way of a voltage divider circuit,which may include a plurality of resistors R30, R31, R33 and R34, R35,R38, R39, R41, R43, R44, R46, R48, R49, R51, R52, R54, R57, R58, R59,R61, as well as a plurality of operational amplifiers U4A, U4B, U4C andU4D. For brevity, only the amplifier circuit for the battery cell 28 isdescribed. The other amplifier circuits operate in a similar manner. Inparticular, for the battery cell 28, the charging current through thebattery cell 28 is dropped across the resistor R37. That voltage drop isapplied across a non-inverting input and inverting input of theoperational amplifier U4D.

The resistors R31, R33, R34, and R35 and the operational amplifier U4Dform a current amplifier. In order to eliminate the off-set voltage, thevalue of the resistors R33 and R31 value are selected to be the same andthe values of the resistors R34 and R35 value are also selected to bethe same. The output voltage of the operational amplifier U4D=voltagedrop across the resistor R37 multiplied by the quotient of the resistorvalue R31 resistance value divided by the resistor value R34. Theamplified signal at the output of the operational amplifier U4D isapplied to the microprocessor 26 by way of the resistor R30. Theamplifier circuits for the other battery cells 30, 32, and 34 operate ina similar manner.

Charge Termination Techniques

The battery charger in accordance with the present invention canimplement various charge termination techniques, such as temperature,pressure, negative delta, and peak cut-out techniques. These techniquescan be implemented relatively easily by program control and are bestunderstood with reference to FIG. 2. For example, as shown, threedifferent characteristics as a function of time are shown for anexemplary nickel metal hydride (NiMH} battery cell during charging. Inparticular, the curve 40 illustrates the cell voltage as a function oftime. The curves 42 and 44 illustrate the pressure and temperaturecharacteristics, respectively, of a NiMH battery cell under charge as afunction of time.

In addition to the charge termination techniques mentioned above,various other charge termination techniques the principles of theinvention are applicable to other charge termination techniques as well.For example, a peak cut-out charge termination technique, for example,as described and illustrated in U.S. Pat. No. 5,519,302, herebyincorporated by reference, can also be implemented. Other chargetermination techniques are also suitable.

FIG. 2 illustrates an exemplary characteristic curve 40 for an exemplaryNiMH or NiCd battery showing the relationship among current, voltage andtemperature during charge. More particularly, the curve 40 illustratesthe cell voltage of an exemplary battery cell-under charge. In responseto a constant voltage charge, the battery cell voltage, as indicated bythe curve 40, steadily increases over time until a peak voltage valueV_(peak) is reached as shown. As illustrated by the curve 44, thetemperature of the battery cell under charge also increases as afunction of time. After the battery cell reaches its peak voltageV_(peak). continued charging at the increased temperature causes thebattery cell voltage to drop. This drop in cell voltage can be detectedand used as an indication that the battery's cell is fully charged. Thischarge termination technique is known as the negative delta V technique.

As discussed above, other known charge termination techniques are basedon pressure and temperature. These charge termination techniques relyupon physical characteristics of the battery cell during charging. Thesecharge termination techniques are best understood with respect to FIG.2. In particular, the characteristic curve 42 illustrates the internalpressure of a NiMH battery cell during charging while the curve 44indicates the temperature of a NiMH battery cell during testing. Thepressure-based charge termination technique is adapted to be used withbattery cells with internal pressure switches, such as the Rayovacin-cell charge control (I-C³)¹. NiMH battery cells, which have aninternal pressure switch coupled to one or the other anode or cathode ofthe battery cell. With such a battery cell, as the pressure of the cellbuilds up due to continued charging, the internal pressure switch opens,thus disconnecting the battery cell from the charger. (I-C³) is atrademark of the Rayovac Corporation.

Temperature can also be used as a charge termination technique. Asillustrated by the characteristic curve 44, the temperature increasesrather gradually. After a predetermined time period, the slope of thetemperature curve becomes relatively steep. This slope, dT/dt may beused as a method for terminating battery charge.

The battery charge in accordance with the present invention can alsoutilize other known charge termination techniques. For example, in U.S.Pat. No. 5,519,302 discloses a peak cut-out charge termination techniquein which the battery voltage and temperature is sensed. With thistechnique, a load is attached to the battery during charging. Thebattery charging is terminated when the peak voltage is reached andreactivated as a function of the temperature.

Software Control

FIGS. 3A-3E illustrate exemplary flow-charts for controlling the batterycharger in accordance with the present invention. Referring to the mainprogram, as illustrated in FIG. 3A, the main program is started uponpower-up of the microprocessor 26 in step 50. Upon power-up, themicroprocessor 26 initializes various registers and closes all of theFETs Q12, Q13, Q14, and Q15 in step 52. The microprocessor 26 also setsthe pulse-width of the PWM output of the regulated 24 to a nominalvalue. After the system is initialized in step 52, the voltages acrossthe current sensing resistors R37, R45, R53, and R60 are sensed todetermine if any battery cells are currently in any of the pockets instep 54. If the battery cell is detected in one of the pockets, thesystem control proceeds to step 56 in which the duty cycle of the PWMout-put of the regulator 24 is set. In step 58, a charging mode isdetermined. After the charging mode is determined, the microprocessor 26takes control of the various pockets in step 60 and loops back to step54.

A more detailed flow-chart is illustrated in FIG. 38. Initially, in step50, the system is started upon power-up of the microprocessor 26. Onstart-up, the system is initialized in step 52 as discussed above. Asmentioned above, the battery charger in accordance with the presentinvention includes two or more parallel connected charging circuits.Each of the charging circuits includes a switching device, such as aMOSFETs Q12, Q13, Q14, or Q15, serially coupled to the batteryterminals. As such, each charging circuit may be controlled by turningthe MOSFETs on or off, as indicated in step 66 and discussed in moredetail below. In step 68, the output voltage and current of theregulator 24 is adjusted to a nominal value by the microprocessor 26.After the regulator output is adjusted, a state of the battery cell ischecked in step 70. As mentioned above, various charge terminationtechniques can be used with the present invention. Subsequent to step70, the charging current is detected in step 72 by measuring thecharging current dropped across the current sensing resistors R37, R45,R53, or R60.

One or more temperature based charge termination techniques may beimplemented. If so, a thermistor may be provided to measure the externaltemperature of the batter cell. One such technique is based on dT/dt.Another technique relates to temperature cutoff. If one or more of thetemperature based techniques are implemented, the temperature ismeasured in step 74. If a dT/dt charge termination technique isutilized, the temperature is taken along various points along the curve44 (FIG. 2) to determine the slope of the curve. When the slope isgreater than a predetermined threshold, the FET for that cell is turnedoff in step 76.

As mentioned above, the system may optionally be provided with negativedelta V charge termination. Thus, in step 78, the system may constantlymonitor the cell voltage by turning off all but one of the switchingdevices Q12, Q13, Q14, and Q15 and measuring the cell voltage along thecurve 40 (FIG. 2). When the system detects a drop in cell voltagerelative to the peak voltage Vsen, the system loops back to step 66 toturn off the switching device Q12, Q13, Q14, and Q15 for that batterycell.

As mentioned above, a temperature cut-out charge termination techniquemay be implemented. This charge termination technique requires that thetemperature of the cells 28, 30, 32 and 34 to be periodically monitored.Should the temperature of any the cells 28, 30, 32 and 34 exceed apredetermined value, the FET for that cell is turned off in step 80. Instep 82, the charging time of the cells 28, 30, 32, and 34 isindividually monitored. When the charging time exceeds a predeterminedvalue, the FET for that cell is turned off in step 82. A LED indicationmay be provided in step 84 indicating that the battery is being charged.

FIG. 3C illustrates a subroutine for charging mode detection. Thissubroutine may be used to optionally indicate whether the batterycharger 20 is in a “no-cell” mode; “main-charge” mode;“maintenance-charge” mode; an “active” mode; or a “fault” mode. Thissubroutine corresponds to the block 58 in FIG. 3A. The system executesthe charging mode detection subroutine for each cell being charged.Initially, the system checks in step 86 the open-circuit voltage of thebattery cell by checking the voltage at terminal Vsen of themicroprocessor 26. If the open-circuit voltage is greater than or equalto a predetermined voltage, for example, 2.50 volts, the system assumesthat no battery cell is in the pocket, as indicated in step 88. If theopen-circuit voltage is not greater than 2.50 volts, the system proceedsto step 90 and checks whether the open-circuit voltage is less than, forexample, 1.90 volts. If the open circuit voltage is not less than 1.90volts, the system indicates a fault mode in step 92. If the open-circuitvoltage is less than 1.90 volts, the system proceeds to step 94 andchecks whether the open-circuit voltage is less than, for example, 0.25volts. If so, the system returns an indication that the battery chargeris in inactive mode in step 96. If the open-circuit voltage is not lessthan, for example, 0.25 volts, the system proceeds to step 98 and checkswhether a back-up timer, is greater than or equal to, for example, twominutes. If not, the system returns an indication that battery charger20 is in the active mode in step 96. If the more than, for example, twominutes has elapsed, the system checks in step 100 whether the batterycell voltage has decreased more than a predetermined value, for example,6.2 millivolts. If so, the system returns an indication in step 102 of amaintenance mode. If not, the system proceeds to step 104 and determineswhether the back-up timer is greater or equal to a maintenance timeperiod, such as two hours. If not, the system returns an indication instep 106 of a main charge mode. If more than two hours, for example haselapsed, the system returns an indication in step 102 of a maintenancemode.

FIG. 30 illustrates a subroutine for the PWM duty cycle control. Thissubroutine corresponds to block 56 in FIG. 3A. This subroutine initiallychecks whether or not a cell is present in the pocket in step 108 asindicated above. If there is no cell in the pocket, the duty cycle ofthe PWM is set to zero in step 110. When there is a battery cell beingcharged, the PWM output current of the regulator 24 is sensed by themicroprocessor 26 by way of sensing resistor R11. The microprocessor 26uses the output current of the regulator 24 to control the PWM dutycycle of the regulator 24. Since the total output current from theregulator 24 is dropped across the resistor R11, the system checks instep 111 whether the voltage Vsen is greater than a predetermined value,for example, 2.50 volts in step 111. If so, the PWM duty cycle isdecreased in step 115. If not, the system checks whether the totalcharging current for four pockets equal a predetermined value. If so,the system returns to the main program. If not, the system checks instep 114 whether the charging current is less than a preset value. Ifnot, the PWM duty cycle is decreased in step 115. If so, the PWM dutycycle is increased in step 116.

The pocket on-off subroutine is illustrated in FIG. 3E. This subroutinecorresponds to the block 60 in FIG. 3A. Initially, the system checks instep 118 whether the battery cell in the first pocket (i.e. channel 1)has been fully charged. If not, the system continues in the main programin FIG. 3A, as discussed above. If so, the system checks in step 120which channels (i.e pockets) are charging in order to take appropriateaction. For example, if channel 1 and channel 2 are charging and channel3 and channel 4 are not charging, the system moves to step 122 and turnsoff channel 3 and channel 4, by turning off the switching devices Q14and Q15 and moves to step 124 and turns on channel 1 and channel 2, byturning on the switching device Q12 and Q13.

The channels refer to the individual charging circuits which include theswitching devices Q12, Q13, Q14, and Q15. The channels are controlled byway of the switching devices Q12, Q13, Q14 or Q14 being turned on or offby the microprocessor 26.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. Thus, it is to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described above.

What is claimed and desired to be secured by a Letters Patent of theUnited States is:
 1. A multiple cell battery charger comprising: aregulator for receiving a predetermined input voltage and supplying aregulated supply of DC voltage; a plurality of charging circuits, eachcharging circuit formed to charge at least one battery cell, saidplurality of charging circuits electrically coupled to each other in aparallel relationship forming a parallel circuit, said parallel circuitbeing electrically coupled to said regulator and connected in parallelwith each other, each charging circuit comprising; a pair of terminalsfor coupling to said at least one battery cell; a switching deviceserially coupled to said pair of battery terminals for selectivelyconnecting and disconnecting said pair of terminals from said regulator;and a current sensing device for sensing the charging current applied tosaid at least one battery cell, wherein said current sensing device,said switching device and said pair of terminals are all connected inseries forming said charging circuit; a microprocessor operativelycoupled to said charging circuits for selectively monitoring the voltageacross the pair of terminals in each charging circuit independently andselectively controlling the switching device so as to disconnect saidpair of terminals in an individual charging circuit when said at leastone battery cell in that individual charging circuit reaches apredetermined voltage.
 2. The original multiple cell battery charger asrecited in claim 1, wherein said regulator is a switching regulator witha selectable pulse width modulated output signal.
 3. The multiple cellbattery charger as recited in claim 2, selectable pulse width modulated(PWM) output signal is under the control of said microprocessor whichvaries the pulse width of said PWM output signal as a function of themagnitude of said the total charging current supplied by said regulatorforming a closed feedback loop.
 4. The multiple cell battery charger asrecited in claim 1, wherein said predetermined input voltage to saidregulator is AC.
 5. The multiple cell battery charger as recited inclaim 1, wherein said predetermined input voltage is DC.
 6. The multiplecell battery charger as recited in claim 1, wherein said switchingdevice is a field effect transistor having gate, drain and sourceterminals, wherein said drain and source terminals are serially coupledto said individual cell current sensing resistor and said battery cellterminals and said gate terminal is electrically coupled to saidregulator.
 7. The multiple cell battery charger as recited in claim 1,wherein said regulator is an integrated circuit.
 8. A multiple cellbattery charger comprising: a regulator for receiving a predeterminedinput voltage and supplying a regulated supply of DC voltage; aplurality of charging circuits, each charging circuit formed to chargeat least one battery cell, said plurality of charging circuitselectrically coupled to each other in a parallel relationship forming aparallel circuit, said parallel circuit being electrically coupled tosaid regulator and connected in parallel with each other, each chargingcircuit comprising; a pair of terminals for coupling to said at leastone battery cell; a switching device serially coupled to said pair ofbattery terminals for selectively connecting and disconnecting said pairof terminals from said regulator; and a current sensing device forsensing the charging current applied to said at least one battery cell,wherein said current sensing device, said switching device and said pairof terminals are all connected in series forming said charging circuit;a processing device operatively coupled to said charging circuits forselectively monitoring the voltage across the pair of terminals in eachcharging circuit independently and selectively controlling the switchingdevice so as to disconnect said pair of terminals in an individualcharging circuit when said at least one battery cell in that individualcharging circuit reaches a predetermined voltage.